The present invention relates to a passive optical network, and, more particularly, is directed to use of multimode or multifrequency lasers as light sources in a wavelength division multiplexed passive optical network.
As defined in Frigo, xe2x80x9cA Survey of Fiber Optics in Local Access Architecturesxe2x80x9d, in Kaminow et al., ed., Optical Fiber Telecommunications IIIA, Academic Press, 1997, pp 461-522, the disclosure of which is hereby incorporated by reference, the term passive optical network (PON) refers to a configuration, as shown in FIG. 1, having a central office with a host digital terminal (HDT) coupled via optical fibers to remote nodes which are, in turn, coupled via optical fibers to subscriber terminals. The central office and terminals exchange information via light signals carried by the optical fibers. The xe2x80x9cpassivexe2x80x9d aspect of the PON is due to the processing, at the remote nodes, of signals between the central office and terminals in an entirely optical manner, that is, without conversion to electronic signals. The optical fibers may or may not amplify the signals which they carry.
One type of PON is the wavelength division multiplexed (WDM) passive optical network (PON). In a WDM PON, a significant component""at the remote node is a wavelength division multiplexer, which serves to combine, or multiplex, a plurality of optical signals each having a distinct wavelength and having respective data rates into an optical transmission having a data rate which is at least the sum of the respective data rates, and also serves to separate, or demultiplex, a single optical transmission into a plurality of optical signals each having a distinct wavelength and having respective data rates lower than the data rate of the single optical transmission. Different optical wavelengths are used between the central office and the respective terminals, so that the WDM PON functions as a logical star configuration.
A waveguide grating router (WGR) is a type of WDM. In a WGR, shown in FIG. 2, incoming light is split into spectral constituents and output to a set of fibers. A WGR has a routing property and a periodicity property.
As explained in Frigo, the routing property of a WGR means that for a set of input ports, i1, i2 . . . in, and a set of output ports, o1, o2 . . . on, an incoming signal at one of the input ports having frequency fk, 1xe2x89xa6kxe2x89xa6n, selects its own routing due to its frequency. Let the input port be ij and the output port be op, 1xe2x89xa6jxe2x89xa6n, 1xe2x89xa6pxe2x89xa6n. For an incoming signal fk at input port ij, the output signal will be at output port op, with p=(j+(kxe2x88x921)) modulo n.
The periodicity property of a WGR means that an input signal k at a frequency exceeding n exits at an output port given by k modulo n. That is, the periodicity property is expressed as a modulo function, corresponding to a sort of xe2x80x9cwrap aroundxe2x80x9d, rather than having the input signal be lost. As explained in Frigo, the periodicity property effectively samples a broadband incoming signal at many points for each port.
As described in U.S. Pat. No. 5,680,234 (Darcie et al.), having a common assignee with the present invention, the disclosure of which is hereby incorporated by reference, signals can be broadcast from a central office to remote nodes using a WDM PON. Henceforth, a PON containing a WDM at the remote mode is referred to herein as a WDM PON, regardless of whether the multiplexing or demultiplexing capability of WDM is used.
In the upstream direction, from the subscriber terminal to the central office, use of a light emitting diode (LED) as the light source at the subscriber terminal has been proposed to avoid the need for a frequency-specified laser or a modulator at the terminal""s optical network unit, thereby reducing deployment costs. In this proposal, the output of a LED is directly modulated with the upstream information and is spectrally sliced into different optical bands at the WGR.
In the downstream direction, from the central office to a terminal, a lightsource proposed for use at the central office transmitter is a multi-frequency laser, including laser arrays or tunable lasers, which is wavelength controlled so that the frequencies emitted by the laser line up with, or match, the frequencies of the WGR and channels. A difficulty in using uncooled multi-frequency lasers as optical sources is that the frequency of the light drifts, and so the signal frequencies become misaligned with the WGR passband frequencies. Furthermore, the WGR may have an environmental sensitivity: as the temperature changes, the spectral location of its passbands is likely to move, which is a source of misalignments. The drift problem can be corrected by controlling the laser""s temperature, but this raises the cost of the laser. A problem with temperature controlled transmitters is that they are too expensive to be used at each terminal, but they are economically feasible at the central office.
Alternatively, Iannone, Frigo and Darcie, xe2x80x9cWDM passive optical network architecture with bidirectional optical spectral slicing,xe2x80x9d Optical Fiber Conference ""95 Technical Digest, pages 51-53, have proposed using a broadband source, such as a 1.3 xcexcm LED, to broadcast downstream information. Use of an LED as an optical source has disadvantages, such as the LED cannot be modulated as fast as a laser can be modulated, and the LED generates signals with less optical power than the signals generated by a laser.
Thus, there are opportunities to improve light sources in WDM PONs.
In accordance with an aspect of this invention, there is provided a passive optical network (PON) comprising an optical device for producing light at a plurality of optical frequencies, an optical receiver for receiving light produced by the optical device, and a wavelength division multiplexer for transmitting the light produced by the optical device to the optical receiver, the wavelength division multiplexer having passbands centered at respective center frequencies, the center frequencies being different than the optical frequencies.
In aspects of the invention, the optical device is a multifrequency laser, an array of lasers, or a multimode laser, such as a Fabry-Perot laser, driven in such a manner as to increase the number of usable modes. The temperature of the optical device is either uncontrolled or locally controlled to reduce costs, that is, the laser""s temperature is not specifically set to align the laser""s modes with the passbands of the wavelength division multiplexer.
In other aspects of the invention, the wavelength division multiplexer transmits light produced by the optical device in each of its passbands, and the spacing between the optical frequencies is less than the spacing between the center frequencies. The wavelength division multiplexer may be a waveguide grating router having a free spectral range less than the frequency range of the optical frequencies.
In a further aspect of the invention, the optical device is associated with a first subscriber, there is at least one additional optical device associated with at least one additional subscriber, and there is means for equalizing the light from different optical devices, such as a prebias current supplied to one of the multimode lasers.
It is not intended that the invention be summarized here in its entirety. Rather, further features, aspects and advantages of the invention are set forth in or are apparent from the following description and drawings.